For all accretion geometries, the supersonic infalling matter makes up the accretion flowIn
[1]}, [2]}, this whole region (out to \({R_{\textnormal {acc}}}\) ) is often called the “preshock region”, meant as a synonym. However, for clarity, we keep here the term “preshock” for the layers immediately before (i.e. upstream of) the shock, as is more common in the literature.
before hitting the planet's surface, defined as the location of the radiative hydrodynamical shock [1]}, [2]}.
This is illustrated in Figure REF .
Immediately below the hydrodynamical shock is the postshock region proper, a spatially thin “settling zone” and cooling region. There, the gas is subsonic and gradually slows down, reaching hydrostatic equilibrium at depths, where
the structure is that of a non-accreting planet. The postshock region consists of
the layers close to but below the shock, especially the thin cooling region. Note that an accreting planet does not have an atmosphere in the classical sense (for an isolated object), or does so at most only on the parts of its surface that are not accreting.
